net: dsa: mv88e6xxx: Preserve priority when setting CPU port.

net: dsa: mv88e6xxx: Preserve priority when setting CPU port.

BugLink: https://bugs.launchpad.net/bugs/1861739
[ Upstream commit d8dc2c9676e614ef62f54a155b50076888c8a29a ]

The 6390 family uses an extended register to set the port connected to
the CPU. The lower 5 bits indicate the port, the upper three bits are
the priority of the frames as they pass through the switch, what
egress queue they should use, etc. Since frames being set to the CPU
are typically management frames, BPDU, IGMP, ARP, etc set the priority
to 7, the reset default, and the highest.

Fixes: 33641994a676 ("net: dsa: mv88e6xxx: Monitor and Management tables")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>